Magnetic resonance imaging (“MRI”) is the optimal imaging modality for the prostate and surrounding critical organ structures. However, with MRI, the standard titanium radioactive seeds, strands of seeds, and needle tracks appear as black holes (negative contrast) and cannot be accurately localized within the prostate and periprostatic tissue. Without adequate localization of the radioactive seeds, MRI-based dosimetry is inaccurate, and therefore, MRI is not used in treatment planning, treatment delivery, or postimplant treatment quality evaluation. For example, brachytherapy titanium encapsulated seeds appear as negative contrast as well as any spacers and needle tracks. In turn, the seeds limit the ability to perform functional imaging of the prostate as the dosimetry cannot be accurate without positive identification of the seed.
MRI-CT fusion has been shown to improve postimplant quality assessment over CT alone, but this combined imaging approach has not been translatable to the community setting owing to inadequacies of fusing caused by imaging with different bladder and rectal filling, prostate volumetric differences between imaging modalities, and difficulties fusing the negative contrast of the seeds, strands of seeds, and needle tracks with the seeds visualized on CT scan. Crook, J., et al., Interobserver Variation Inpostimplant Computed Tomography Contouring Affects Quality Assessment of Prostatebrachytherapy, Brachytherapy, 2002, 1(2):66-73.
The consequence of the current inadequate ultrasound and CT imaging is subjective dosimetric evaluation and poor quality assurance during and after brachytherapy. Poor-quality implants are of critical clinical importance because they lead to decreased cure rates and increased side effects after treatment. Therefore, there is a critical need for national standardization of prostate brachytherapy dosimetry. This effort may be achieved through the design of seed implants of improved design that incorporate high contrast imaging capabilities.
Furthermore, novel MRI pulse sequences and protocols have been inadequate in identifying all of the implanted radioactive seeds and is not an adequate replacement of CT for evaluating dosimetry. For example, a recent article by Bloch et al. have proposed the use of high-resolution contrast-enhanced MRI (HR-CEMRI) with an endo-rectal coil as a complement to a T2-weighted MRI data set for MRI as a single imaging modality for the postimplant dosimetric evaluation. However, 12% (CEMRI) and 29% (T2-weighted MRI) of the seeds were missing which can lead to large dose uncertainties and should not be tolerated for accurate clinical dose reporting.